Directional drilling has proven useful in facilitating production of fluid, e.g. hydrocarbon-based fluid, from a variety of reservoirs. In many applications, a vertical wellbore is drilled, and casing is deployed in the vertical wellbore. One or more windows are then milled through the casing to enable drilling of lateral wellbores. Each window formed through the casing is large enough to allow passage of components, e.g. passage of a bottom hole assembly used for drilling the lateral wellbore and of a liner for lining the lateral wellbore. The bottom hole assembly may comprise a variety of drilling systems, such as point-the-bit and push-the-bit rotary drilling systems.
However, conventional wellbore departure and drilling systems are designed in a manner which generally requires multiple downhole trips. For example, a window milling bottom hole assembly may initially be run downhole to create an exit path in the existing casing of the vertical wellbore. The window milling bottom hole assembly also may be employed to drill a rathole of sufficient size for the next drilling assembly. In a subsequent trip down hole, a directional drilling bottom hole assembly is run to extend the rathole and to drill laterally to a desired target and to thus create the lateral wellbore.